Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
OH + CH 4 CH3 + H 2 O (35)
As a result, the global CH 4 distribution adjusts slowly to higher OH levels, and a new steady-state is established in which CH 4 concentrations are reduced slightly. This
adjustment in CH 4 itself drives a further increase in OH levels, with the result that CH 4 concentrations build up more slowly over 10-15 years. These indirect or
feedback processes have been described in some detail in previous IPCC (1995, 1996) reports.
As quantified in the following section, aircraft NOx emissions produce an increase in the total global inventory of OH of about 2%, which should be reflected in a
corresponding change in CH4 loss rate (IPCC, 1995, 1996). The CH4 chemical feedback will then amplify this change in loss rate, producing a decrease in CH4 concentrations that is about 1.4 times the change in loss rate. CH4 concentrations should then decrease by about 3%. These reduced CH4 burdens will then decrease
tropospheric ozone production, although the effect may be considered negligible (Fuglestvedt et al., 1999). Because these readjustments take place over 10-15 years,
they are exceedingly difficult to represent fully in global 3-D models, though they have been fully explored in 2-D models (Fuglestvedt et al., 1996; Johnson and
Derwent, 1996).
The stratospheric modeling tools described in Chapter 4 indicate that supersonic aircraft flying in the LS may lead to stratospheric ozone depletion. Under conditions of
stratospheric ozone depletion, there is greater penetration of solar ultraviolet radiation through the stratosphere. N2O transported up from the troposphere therefore
has higher photolysis rates lower in the stratosphere, leading to a shorter photolysis lifetime. The global N2O distribution will then slowly adjust over decades to the
increase in stratospheric destruction, leading to decreased N2O concentrations, assuming constant emissions. Because N2O is an important greenhouse gas, there is
an indirect radiative forcing impact of supersonic aircraft over 50-200 years. This indirect impact is relatively straightforward to take into account using modeling tools
described in Chapter 4.
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